The other day I noticed an issue with one of my Agilent 34401A’s: I measured a constant DC voltage and after a few minutes the measured value would start dropping and become unstable. Then I got curious and checked the two-wire resistance mode. Same story, maybe worse.
A word of caution: High voltages found inside the DMM can cause damage, severe injury or death. If you are not authorized or qualified to work on such devices, don’t do it. I don’t take any responsibility for actions you take or the results of these actions. You do everything at your own risk.
Without checking the current range, which might have been a clever thing to do, I immediately suspected a contact issue with the front/rear switch, so I tested the units rear connectors. If the problem had then disappeared, I would have concluded that the front/rear switch was causing the error – in particular the switch contacts for the front connectors. Initially the measurement seemed stable, but after a while the problem resurfaced. So I opened the unit to have a closer look. I applied a (low) voltage, powered the unit on and waited until the readings became unstable.
![](https://sebastianharnisch.de/wp-content/uploads/2023/03/20220111_190547822_iOS-1024x640.jpg)
With the help of the service manual – I followed the signal path until I detected an erroneous voltage: after relay K101 the voltage became unstable. The control signal for the relay was present (very unlikely that a problem here would result in the symptoms described), however manually shorting the relay contact temporarily immediately stabilized the meter’s reading. That left only one conclusion: K101 is the culprit.
According to the table found in the service manual relay K101 is closed regardless of the function/range selection. However, it opens momentarily during function/range switching. This might make it seem plausible that the meter showed correct values for a while right after selecting activating a function (i. e. open and then close K101 again).
K101 is a low thermal emf relay with a rather high voltage rating. The service manual lists it as “Relay-Reed 2A 500 Vdc 5 Vdc-Coil”, part number 0490-1914.
![](https://sebastianharnisch.de/wp-content/uploads/2023/03/20230312_005526519_iOS-1024x677.jpg)
An EEVblog forum search reveals that this is indeed a fairly common issue. Since there are two different relay manufacturers for part number 0490-1914 it would be interesting to know whether there are differences in reliability. My faulty unit used a COTO relay (red package), whereas my other 34401A uses a MEDER relay (blue package).
![](https://sebastianharnisch.de/wp-content/uploads/2023/03/20211217_164402779_iOS-1024x640.jpg)
The more difficult task was to find a replacement for it, which is pretty much unobtanium at this point. With help of the parametric search of a big distributor and also hints/confirmation from some eevblog forum members I finally ordered a COTO 3501-05-511 low thermal emf reed relay which shares a similar pin layout. Similar means that I simply had to cut off one pin. There is a catch though: The new relay is only rated for 200VDC, explaining the smaller size (likely only one reed contact vs two in series in the original).
![](https://sebastianharnisch.de/wp-content/uploads/2023/03/20220111_191056695_iOS-1024x640.jpg)
From now on, on the rare occasion that I have to measure voltages significantly higher than the 200V, I should use the other 34401A. And if that’s not an option: Pre-selection of the function and range can keep K101 closed – at least for the most part.
Later I discovered this interesting post: https://www.eevblog.com/forum/repair/recovery-of-the-relay-k101(0490-1914)-in-hpagilent-34401a/, suggesting that it might be possible to restore functionality by “reflowing” an internal solder joint. Not so sure about it… I did reflow the relay pins, but this would certainly not have been enough to reflow any internal solder connection. Anyways…
That’s one Agilent 34401A saved from the landfill.